Generally, a coaxial connector is used to connect signal lines for transmitting a high-speed (radio frequency) electric signal. An inner conductor serving as a signal line is provided in a central part of a coaxial connector, and an outer conductor serving as a grounding line is provided to surround the inner conductor. A dielectric material is filled between the inner conductor and the outer conductor. An outer diameter of the inner conductor and an inner diameter of the outer conductor are set to predetermined diameters so as to match a specific impedance (for example, 50Ω).
In the above-mentioned coaxial connector, there is a cutoff frequency fc at which a signal having a frequency higher than a fixed frequency cannot be transmitted. The cutoff frequency fc is determined by the outer diameter of the inner conductor, the inner diameter of the outer conductor, and a specific dielectric constant of the dielectric material filled between the inner conductor and the outer conductor. The cutoff frequency fc becomes higher as the diameters become smaller and the specific dielectric constant becomes lower. Accordingly, in order to transmit a radio frequency signal, it is necessary to make the diameter of the coaxial connector small and make the specific dielectric constant of the filled dielectric material low. Generally, in order to obtain a radio frequency transmission band of about more than 60 GHz, the outer diameter of the inner conductor is reduced to about 1 mm and an air (ε r=1.0) is used as a dielectric material.
In recent years, miniaturization and speeding up have progressed in measuring instruments and optical transmission and reception devices that handle a high-speed (radio frequency) electric signal. With such a progress, there is a demand for miniaturizing coaxial connectors used for those devices are required. Although connectors having a screw-type connecting part, which are represented by a 2.92 mm connector or a 1.85 mm connector, were in popular use, connectors having a push-on type connecting part, such as an SMP connector or an SMPM connector, have become popular with the demand for miniaturization (for example, refer to Non-Patent Document 1).
In many cases, a coaxial connector used for connection between measuring instrument or devices is provided with functions such as a DC block or a frequency filter. The DC block is provided for interrupting a direct current component and to transmit only an alternating current (AC) signal. The frequency filter is provided for attenuating a specific frequency component of a signal.
Specifically, the DC block and the frequency filter are formed by inserting a capacitor in the middle of the inner conductor. For example, it is suggested to divide the inner conductor into a first inner conductor and a second inner conductor and connecting the first and second inner conductors with two flat-plate capacitors located therebetween in series (for example, refer to Patent Document 1). Additionally, it is suggested to divide the inner conductor into a first inner conductor and a second inner conductor while forming surfaces parallel to the axis and connecting the first and second inner conductors with a dielectric material located therebetween (for example, refer to Patent Document 2).
According to the structures of the DC blocks, a strength of a connecting part (a part where the DC block is formed) between the first inner conductor and the second inner conductor is small, and the connecting part may be damaged due to a thermal stress of the inner conductor or the like. Thus, it is suggested to provide a stress relaxation mechanism for absorbing and relaxing a stress in the axial direction (for example, refer to Patent Document 3)
Patent Document 1: U.S. Pat. No. 6,496,353
Patent Document 2: U.S. Pat. No. 7,180,392
Patent Document 3: U.S. Pat. No. 5,576,675
Non-Patent Document 1: U.S. military standard MIL_STD—348A
If a capacitor is interposed in the middle of the inner conductor as mentioned above, it is difficult to equalize an impedance between the capacitor and the outer conductor and an impedance between the inner conductor and the outer conductor. That is, a distance between the inner conductor and the outer conductor, which is set to maintain a predetermined impedance, is changed at the portion of the capacitor, which results in a change in the impedance. Accordingly, an impedance mismatch occurs at the portion where the capacitor is provided, which causes degradation of a radio frequency signal transmission characteristic.
Accordingly, it is desirous to develop a small coaxial connector having a structure in which, even if a capacitor is inserted in a middle of an inner conductor, an impedance mismatch at a portion where the capacitor is provided is suppressed.